1. Field of the Invention
This invention relates to reflective compensating arrangements for reducing transmission line noise effects in electrical circuits (e.g. printed circuit boards and cards) which route pulse signals to multiple loads (representing signal receiving circuits or devices) through complex signal conduction networks.
2. Background Discussion and Related Art
As technology advances, rise times of pulse signals tend to shorten and approach signal propagation times between signal drivers/sources and receiving loads. Consequently, small lengths of signal conductors can act like analog transmission lines, producing reflections which may distort received signals (producing effects such as ringing, bouncing, overshoot, etc.); especially in signal conducting networks originating at the source and splitting into multiple branches en route to the loads. Such distortion, combined with other sources of noise (e.g. cross-talk between conductors), may produce faulty operations in circuits which otherwise appear to have satisfactory design specifications. Accordingly, designers of digital logic and devices for high technology packaging (e.g. on printed circuit cards or boards) have become increasingly concerned with such transmission line effects, especially with respect to circuits having critical timing requirements for signal reception.
U.S. Pat. No. 5,175,515, granted Dec. 29, 1992 to M. G. Abernathy et al, discloses a technique for routing pulse signals on printed circuit boards (PCB's) which relies essentially on configuring signal loads into branching tree formations extending symmetrically from a signal source, and if necessary appending "lossy" ac or dc terminators at or near a last load in each branch (ostensibly to absorb energy and reduce signal reflections in each branch).
However, it is known that end terminators alone can not eliminate excessive distortions due to transmission line reflections, and it is recognized presently that such terminators may not be effective even when combined with the branch balancing technique suggested in the Abernathy et al patent. Furthermore, topological packaging restrictions may make branch balancing impractical or very difficult (and therefore costly) to implement.
Even more significant, it is recognized presently that signal distortions associated with transmission line effects are due to both reflections and re-reflections of signals, rather than to reflections only, and that taking steps to suppress or reduce re-reflections may be more useful for reducing aggregate distortion, and easier and less costly to implement, than any of the techniques suggested by Abernathy et al. Ancillary to this last observation is a recognition of a need for improved methods for analyzing signal reflections in transmission lines and compensating for their effects.
Although analysis of signal reflections in single line transmission paths may be rather straightforward, the analysis quickly becomes tedious or even unmanageable for complex transmission paths with multiple branches. A time-based analysis of any point in a transmission network with multiple branches reveals superpositions of incident and reflected signal components at the respective point, but in an algebraically additive form in which information about origins and polarizations of individual signal components is unavailable. Accordingly, it is recognized presently that time-based analyses are unsuitable for dealing with reflection problems of the type which the present invention seeks to resolve.